LETTER

Applied Physics Express 12, 022005 (2019)https://doi.org/10.7567/1882-0786/aaf416

Proposal of external modulation scheme for fiber-optic correlation-domain
distributed sensing
Kohei Noda* , Giwoon Han, Heeyoung Lee , Yosuke Mizuno , and Kentaro Nakamura
Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan*

E-mail: knoda@sonic.pi.titech.ac.jp

Received October 17, 2018; accepted November 23, 2018; published online January 10, 2019
All the previous configurations of optical correlation-domain reflectometry (OCDR) have been based on the direct modulation of laser driving
current. However, some lasers are not designed for high-speed large-amplitude modulation, and the modulation amplitude and frequency aremutually dependent. In this work, to mitigate these drawbacks, we propose an external modulation scheme for OCDR. After confirming its basic
operation, we show that a non-specially designed laser can be employed. We also clarify that the interdependence of the modulation amplitudeand frequency is negligibly small in this scheme, which is not the case for the conventional direct modulation scheme.
© 2019 The Japan Society of Applied Physics

O

ne of the useful techniques for monitoring thesoundness of optical components, modules, andﬁber networks is optical reﬂectometry, which has
been used to perform multiplexed sensing,1,2) distributed
sensing,3–5) and optical coherence tomography.6–8) To detect
low-quality connections (or splices) and other reﬂection
points along an optical ﬁber, optical time-domain reﬂectometry (OTDR)9–13) and optical frequency-domain reﬂectometry (OFDR)14–18) have been widely studied. However,OTDR suffers from a relatively low spatial resolution
determined by the optical pulse width and a long measurement
time, and OFDR generally suffers from phase ﬂuctuations
caused by environmental disturbance; in addition, random
access to sensing positions is not available in these techniques.
One of the methods for mitigating these shortcomings is opticalcorrelation (or coherence)-domain reﬂectometry (OCDR),19–30)
which operates based on the synthesis of optical coherencefunctions (SOCF).28) In SOCF, optical frequency is modulated
in a sinusoidal manner21–25) or in a stepwise manner26–28)
including optical frequency comb.29,30) Here, we focus on
SOCF-OCDR based on sinusoidal modulation, which has been
most widely studied owing to its ease of implementation andcost efﬁciency.21–25)
To date, all of the SOCF-OCDR conﬁgurations have been
implemented by directly modulating the driving current of
the laser to modulate the optical output-frequency.21–25)
Although this direct modulation scheme can be relatively
easily implemented, it has following three drawbacks: (i)some lasers are not designed for high-speed large-amplitude
modulation of its driving current (and thus, a suitable laser
needs to be selected to achieve stable operation), (ii) opticalfrequency modulation inevitably accompanies optical power
modulation (and thus, an unintended apodization effect may
deteriorate the performance),31,32) and (iii) the modulation
amplitude and frequency are mutually dependent and it is
difﬁcult to independently control one of the two (and thus,
the actual spatial resolution may largely alter according to thesensing position).
In this work, to mitigate these drawbacks, we propose an
external modulation scheme for SOCF-OCDR and conﬁrm
its basic operation. Subsequently, focusing on the drawbacks(i) and (iii), ﬁrst, we show that a laser not designed for special
use can be employed in this scheme. We then measure themutual dependence of the modulation amplitude and thefrequency in both the direct and external modulation

schemes, and show that, unlike in the direct modulation
scheme (in which the modulation amplitude is sometimes
even doubled by the change in the modulation frequency),
these parameters are almost independent of each other in theexternal modulation scheme.
In SOCF-OCDR,21–25) optical frequency is modulated togenerate periodical correlation peaks along a ﬁber under test
(FUT), enabling selective detection of the reﬂection signal
from an arbitrary position. A distributed measurement isperformed by controlling the modulation frequency fm to scan
one of the correlation peaks along the FUT. The measurement range of the system D is determined by the interval of
the correlation peaks and given by28)
D=

c
,
2nfm

(1 )

where c is the light velocity in vacuum, and n is the refractive
index of the ﬁber core. The spatial resolution Δz is given
by28,33)
Dz @

0.76c
,
pnDf

(2 )

where Δf is the modulation amplitude. According to Eq. (2),
it is clear that, in theory, the spatial resolution is kept constantregardless of the sensing position during distributed measurement (later in this paper, this will be shown not to benecessarily true in practical systems). In conventional SOCFOCDR, frequency-modulated light was generated by directly
modulating the driving current of the laser. However, as thisdirect modulation scheme poses three aforementioned shortcomings, here we propose an external modulation scheme.
A conceptual setup of the external modulation scheme of
OCDR is schematically shown in Fig. 1. The optical parts
after dividing the laser output into two beams are basicallythe same as those of the direct modulation scheme.21–25) In
the external modulation scheme, instead of directly modulating the laser driving current, a double-sideband modulator(DSBM) and an optical band-pass ﬁlter are employed tomodulate the optical frequency. The optical frequency shiftcorresponds to the frequency of the microwave applied froma voltage-controlled oscillator (VCO) to the DSBM. By
controlling the VCO-applied voltage using a function generator, this frequency shift can be swept at high-speed.
To start with, a basic operation of OCDR, i.e., a distributed
reﬂectivity measurement was demonstrated using the external

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Appl. Phys. Express 12, 022005 (2019)

K. Noda et al.

Fig. 1. (Color online) Conceptual schematic of optical correlation-domain
reﬂectometry (OCDR) based on external modulation. AOM: acousto-optic
modulator, DSBM: double-sideband modulator, ESA: electrical spectrum
analyzer, FG: function generator, PD: photo diode, VCO: voltage-controlled
oscillator.

modulation scheme. A distributed-feedback laser diode(NX8563LB, NEC) operating at 1550 nm with a 3 dB linewidth of ∼1 MHz was used. The VCO (HMC733LC4B,
Analog Devices) had a wide output-frequency range from 10
to 21 GHz. The optical band-pass ﬁlter (BVF-300CL, Alnair)
had an edge roll-off of 12 dB GHz−1 and was used to select
only the upper sideband. In the actual experiment, several
optical devices were additionally employed compared with theconceptual setup shown in Fig. 1. An erbium-doped ﬁber
ampliﬁer was inserted in the pump path (output power:∼25 dBm), and a polarization controller (PCTRL) was inserted
in the reference path so that the relative polarization state
between the reﬂected light and the reference light was
optimized. An acousto-optic modulator was not inserted in
the reference path; instead, the foot of the Fresnel reﬂection
spectrum was exploited.24) Using a zero-span function of anelectrical spectrum analyzer (ESA) with a sweep rate of 10 Hz,
the electrical spectral power at 2.4 MHz was transmitted to a
virtual oscilloscope (OSC) in a computer to derive thereﬂectivity distribution along the FUT (see Ref. 24 for thedetail). The resolution and video bandwidths of the ESA were
300 kHz and 1 kHz, respectively. Averaging was performed
128 times on the OSC. The modulation frequency was 3.41–
3.60 MHz, and the modulation amplitude was 0.175 GHz,
which corresponded to the measurement range of ∼30 m andthe spatial resolution of 0.28 m.
The structure of the FUT is depicted in Fig. 2(a). A 4.1,
5.0, 3.0, and 2.1 m long silica single-mode ﬁbers (SMFs)
were sequentially connected using two physical contact (PC)connectors and an angled PC (APC) connector. A bending
loss was artiﬁcially applied in the 2.1 m long SMF to
suppress the extremely large Fresnel-reﬂected signal at theend of the FUT, which was kept open with a PC connector.
The reﬂectivity distribution measured using the external
modulation scheme is shown in Fig. 2(b). The horizontal
axis indicates the relative position from the proximal PC
connector. The vertical axis indicates the relative electrical
power, which was normalized so that the maximal andminimal plots became 1 and 0, respectively. Four peaks
were clearly observed, the positions of which well agreed
with the locations of the connectors. The peak at ∼5.0 m was
smaller than the other peaks, which is natural considering thatthe reﬂected signal from the APC connector was weaker thanthose from the PC connectors. Thus, a basic operation of
OCDR with the external modulation scheme was conﬁrmed(the detailed characterization of the distributed sensing
performance is out of the scope of this paper).

Fig. 2. (Color online) Conﬁrmation of the basic operation of OCDR basedon external modulation. (a) Structure of the ﬁber under test (FUT). APC:
angled physical contact, PC: physical contact. (b) Measured distribution of
the normalized relative reﬂection power along the FUT.

Fig. 3. (Color online) Experimental setups used to characterize (a) the
direct modulation scheme and (b) the external modulation scheme. AC:
alternating current, DC: direct current, PCTRL: polarization controller.

Subsequently, we focus on the third disadvantage (iii) of
the conventional direct modulation scheme, i.e., the naturethat the modulation amplitude and frequency are dependent
on each other and that the actual spatial resolution varies
according to the sensing position. In order to experimentally
evaluate this point in both the direct and external modulation
schemes, we observed the power spectra and investigated therelation between the modulation frequency and amplitude
using setups depicted in Fig. 3(a) (direct modulation scheme)
and Fig. 3(b) (external modulation scheme). In the directmodulation scheme, the output from the modulated laser(NX8563LB, NEC) at ∼1550 nm was heterodyned with theoutput from another tunable laser and the beat spectrum (set022005-2© 2019 The Japan Society of Applied Physics

Appl. Phys. Express 12, 022005 (2019)

K. Noda et al.

Fig. 5. (Color online) Measured dependencies of modulation amplitude Δf
on modulation frequency fm. Red triangles: direct modulation scheme, blue
squares: external modulation scheme.

Fig. 4. (Color online) Power spectra measured at different modulation
frequencies fm. (a) Direct modulation scheme, and (b) external modulation
scheme.

to ∼1 GHz in this experiment) was observed with an ESA.
The amplitude of the AC voltage applied to the laser driving
current (ΔVLD) was set to 2.0 V. The wavelength of thetunable laser (longer than that of the modulated laser) was
adjusted so that the center frequency of the beat spectrum was
located at approximately 1 GHz. The relative polarization
state was optimized using a PCTRL. Averaging was
performed 40 times on the ESA; the other settings were thesame as those for the demonstration of the basic operation
described above. In contrast, in the external modulation
scheme, the output from the single laser at 1550 nm (sameas the aforementioned modulated laser) was divided into two;
one was externally frequency-modulated (in the ﬁgure,the downshift of the central frequency is denoted as
fA = ∼13 GHz) and self-heterodyned with the other, andthen the beat spectrum was observed using an ESA. The
amplitude of the AC voltage applied to the VCO (VVCO) was
set to 2.5 V. Other settings were the same as those for the
direct modulation scheme. In both experiments, the modulation frequency fm was swept from 0.1 to 10.0 MHz (corresponding to the measurement range of OCDR from ∼1 km
down to ∼10 m).
Figure 4(a) shows the optical spectra of the directmodulation scheme when the modulation frequency fm wasvaried. When fm increased from 0.1 to 1.0 MHz, the spectral
width (corresponding to double the modulation amplitude)
decreased, and the spectral power increased. This is natural if
we consider that these spectra are temporally averaged andthat the laser has its own response time. However, when fm
increased further to 2.3 MHz, the spectral width continuously
decreased, but the spectral power decreased. When fm
increased to 10.0 MHz, the spectral width increased, thespectral power decreased, and the central frequency increased. These strange behaviors indicate that this laser is

not suitable for high-frequency large-amplitude modulationuse. In contrast, Fig. 4(b) shows the optical spectra of theexternal modulation scheme when fm was 0.1 and 10.0 MHz.
The shapes of the two spectra agreed well with the theory; thetwo spectra were almost identical from the viewpoints of thespectral width and power (the spectra measured at other fm
values were also the same). This indicates that, in the external
modulation scheme, a laser not designed for special use canbe employed if it satisﬁes such basic speciﬁcations as power,
linewidth, and wavelength. Note that the power of theexternal modulation scheme was lower than that of the directmodulation scheme in this experiment, because the opticalband-pass ﬁlter was used in the former scheme. For practicaluse, it should be ampliﬁed using an optical ampliﬁer.
Finally, for quantitative evaluation, the modulation amplitude Δf (half of the spectral width) was plotted as a functionof fm for both of the modulation schemes (Fig. 5). In the
direct modulation scheme, with increasing fm, Δf wasinitially reduced and then increased. Thus, Δf was found to
be largely dependent on fm in the direct modulation scheme;by changing fm, Δf could be even doubled [corresponding to
two-fold reduction in the spatial resolution (from 37 to
87 mm in this range); such a wide-range sweeping of fm issometimes needed to implement specially conﬁgured OCDR
systems, which exploit multiple correlation peaks simultaneously to achieve longer measurement ranges].34,35) In
contrast, in the external modulation scheme, the ﬂuctuationsof Δf corresponding to the change in fm were within 3% inthis fm range, proving that Δf is not largely dependent on fm(spatial resolution: ∼60 mm in this range). These results
indicate that the interdependence of the modulation frequency/amplitude can be greatly mitigated by the external
modulation scheme.
In conclusion, to tackle the disadvantages of the directmodulation scheme of SOCF-OCDR, we proposed theexternal modulation scheme. After conﬁrming the basic
operation, we showed that an arbitrary laser which is notdesigned for high-frequency large-amplitude modulation canbe used in this scheme. Subsequently, we proved that themodulation frequency and amplitude are almost independentof each other in the external modulation scheme, whichindicates that the spatial resolution is kept constant in the022005-3© 2019 The Japan Society of Applied Physics

Appl. Phys. Express 12, 022005 (2019)

K. Noda et al.

FUT during a distributed measurement. This is not true forthe direct modulation scheme, where the modulation amplitude is even doubled when the modulation frequencychanged. This external modulation scheme is applicable not
only to standard OCDR but also to Brillouin OCDR3) and its
two-end-access conﬁguration, i.e., Brillouin optical correlation-domain analysis.36) To clarify the inﬂuence of the power
modulation on the system performance will be the next
important task. We believe that our proposal will be an
important technique toward implementing high-performanceﬁber-optic correlation-domain sensing systems.
Acknowledgments The authors are indebted to Prof. Hiroyuki Uenohara,
Tokyo Institute of Technology, for his experimental assistance. This work was
supported by JSPS KAKENHI Grant Numbers 17H04930 and 17J07226, and by
a research grant from the Fujikura Foundation.
ORCID iDs Kohei Noda
https://orcid.org/0000-0003-4976-9657
Heeyoung Lee
https://orcid.org/0000-0003-3179-0386 Yosuke Mizuno
https://orcid.org/0000-0002-3362-4720 Kentaro Nakamura
https://orcid.org/
0000-0003-2899-4484

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